This invention relates to actuators and more particularly to hydraulic actuators to facilitate the engagement and disengagement of a motor vehicle clutch.
Hydraulic clutch actuators may comprise either a slave cylinder positioned externally of the clutch housing and operating the clutch release bearing through a lever mechanism extending through the clutch housing or a concentric actuator wherein the actuator is positioned concentrically around the clutch shaft of the vehicle and includes an annular piston connected to the release bearing of the clutch assembly so that sliding movement of the annular piston in the annular pressure chamber of the cylinder operates in known manner to engage and disengage the clutch of the vehicle Since a rotational force is imparted to the piston through the clutch release bearing from the engine drive shaft it is important that means be provided to inhibit this rotation. This rotation can be inhibited for example by key connections between the piston and the cylinder or by the provision of separate locking members engaging the cylinder and piston. However, these arrangements add to the complexity and expense of the actuator since they require additional parts and additional machining. An arrangement is proposed in U.S. Pat. No. 4,526,259 which utilizes the existing coil spring arranged between the piston and the cylinder housing as an antirotation means. However, the arrangement of U.S. Pat. No. 4,526,259 preludes rotation of the piston and cylinder only in one relative direction (for example the direction induced by the rotation of the engine) but is ineffective to preclude relative rotation between the piston and the cylinder in the opposite direction.